An ingredient of gunpowder later step developed (9)
I believe the answer is:
saltpetre
'an ingredient of gunpowder' is the definition.
'later step developed' is the wordplay.
'developed' is an anagram indicator.
'later'+'step'='laterstep'
'laterstep' with letters rearranged gives 'SALTPETRE'.
(Other definitions for saltpetre that I've seen before include "Gunpowder ingredient" , "Constituent of gunpowder" , "Compound" , "Potassium nitrate" , "It's used as a fertilizer and explosive" .)